Defrost control means responsive to speed of evaporator blower



Aug. 20, 1968 H. L. GIWOSKY DEFROST CONTROL MEANS RESPONSIVE TO SPEED OFEVAPORATOR BLOWER Filed April 21, 1967 L 7 /477 095 JZ/z/ask 6%, e y4550mm? United States Patent Oflice Patented Aug. 20, 1968 3,397,550DEFROST CONTROL MEANS RESPONSIVE TO SPEED OF EVAPORATOR BLOWER Harry L.Giwosky, Milwaukee, Wis., assignor to Controls Company of America,Melrose Park, 11]., a corporation of Delaware Filed Apr. 21, 1967, Ser.No. 632,788 12 Claims. (Cl. 62140) ABSTRACT OF THE DISCLOSURE Thisdisclosure is directed to an arrangement for automatically initiatingdefrost of an evaporator coil in a forced air cooling system on thebasis of the speed of the blower motor which causes air flow through thecoil. As the refrigeration cycle proceeds, frost accumulates on theevaporator coil and restricts air flow through the coil. This results ina reduction in the air being moved by the blower thereby decreasing thetorque load on the motor and causing an increase in the speed of themotor driving the blower. A speed responsive control arrangement sensesthis increase in motor speed and at a preselected speed, or in otherwords a control point in the refrigeration cycle, automaticallyinitiates a defrost cycle, the con trol being further effective toautomatically reestablish the refrigeration cycle when defrosting hasbeen completed.

BACKGROUND OF INVENTION (I) Field of inventin.-This invention relates todefrost controls.

(H) Description of prior art.The accumulation of frost on a cooling coileffects air flow through the coil and it has been recognized that thechange in air flow occurring as a result of frost accumulation can beused as a basis for controlling the start of a defrost cycle. In thisconnection, various defrost controls have been proposed which functionon the basis of this change in air flow.

One proposal has placed mechanical sensing elements in the air stream todirectly sense changes in air flow, ex-

amples of such arrangements can be found in Patents Nos. 1,984,053,1,984,054, 2,962,870 and 3,077,747. Another proposal achieved control onthe basis of the change in the amount of current drawn by the blowermotor as a result of the change in air flow. Patents Nos. 2,001,027,2,001,028 and 2,505,201 contain examples of such arrangements. Anothertype of arrangement relied on a pressure responsive arrangement whichused, as a basis of control, the pressure drop which occurs across afrost accumulating member disposed in the air stream, examples of sucharrangements are shown in Patents Nos. 2,992,542 and 3,004,399. For anumber of reasons these prior arrangements have not been entirelysatisfactory, for example they have not possessed the necessaryreliability and/or have required complex circuitry. In this connection,some of the arrangements have required rather involved circuitry tosense the change in air flow and to convert the sensed condition into ausable control signal and others have relied on the proper functioningof mechanical elements exposed directly to the air flow and theenvironment of the evaporator coil.

SUMMARY OF THE INVENTION It has been discovered that the speed of themotor driving the blower which influences air flow through the coolingcoil will vary as the obstruction to air flow through the coil variesand that this variation in motor speed can be used as the basis forcontrol of a defrost cycle. As frost accumulates on the coil the load onthe motor changes due to the change in air flow occasioned by the frostaccumulation, where the blower is drawing air through the coil therewill be a decrease in air flow which results in a corresponding decreasein load on the blower. This decreased load causes an increase in motorspeed. This invention proposes a control arrangement which responds tothis change in speed and initiates a defrost cycle when the motor speedindicates that a desired control point has been reached, i.e. anexcessive amount of frost has accumulated on the coil. Such anarrangement does not require involved circuitry, either to sense thecontrol point or to convert the sensed condition into a usable controlsignal, and it permits the control elements to be arranged remote fromthe air stream and the environment of the cooling coil. Accordingly, thecontrol of this invention affords a simplicity and reliability whichcould not be achieved from prior proposals.

DESCRIPTION OF DRAWINGS FIG. 1 is a semischematic illustration of arefrigeration system and a blower motor speed responsive control andcontrol circuit;

FIG. 2 is a view illustrating the speed responsive switch in its defrostposition;

FIG. 3 is a partial plan view of the switch portion of the control; and

FIG. 4 is a view of the centrifugal weight.

The control arrangement of this invention can be used with aconventional refrigeration system and is illustrated in connection witha system which includes compressor 10, condenser 12, receiver 14,evaporator 16 and valve 18 for controlling flow of refrigerant into theevaporator coil. At this point it should be noted that the controlarrangement of this invention is applicable to refrigeration systems ingeneral but, for convenience, it will be discussed as thoughincorporated in a system supplying cold air to a walk-in cooler or thelike. In such an application air is cooled by passage through theevaporator coil and is delivered to the cooler.

In the illustrated arrangement blower 20 is arranged within a scrollhousing 22 which is in turn connected to housing 24 enclosing evaporatorcoil 16. Air is drawn through evaporator housing 24 over coil 16 andinto scroll housing 22. The air is cooled as it passes over theevaporator coil and is then discharged through scroll housing outlet 26for delivery to the chill area through suitable conduits. The cooler, orchill area, and the conduits for delivering cold air thereto form nopart of this invention and therefore have not been shown and will not bedescribed.

Motor 28 is connected to and drives blower 20 by means of shaft 30. Themotor is of a type such that its speed varies with the torque load onthe motor, preferably the motor is an induction type motor and can be ashaded pole or permanent split capacitor motor. The motor is thussensitive to variations in torque load on its output shaft and, in thisconnection, it is also preferred that blower 22 be of the radial typehaving forward curved blades. This blower arrangement is desirable as itaffords greater sensitivity to variations in torque load.

As the refrigeration cycle progresses condensation collects and freezeson the evaporator coil. The coil frosts up and as it does air passagethrough the coil is obstructed thereby restricting the amount of airwhich can flow through the coil. In this connection a finned evaporatorcoil is generally used to provide optimum heat transfer between the airflow and the coil and such a coil assembly is illustrated in FIG. 1.Frosting and resulting obstruction to air flow is more significant inconnection with the finned evaporator coil but it will be appreciatedthat even without the fins the frost condition and restricted air flowcan occur. As the amount of air flowing through the coil and into thescroll housing 22 decreases the torque load on blower decreases and thespeed of motor 28 experiences a corresponding increase. At this point itshould also be noted that as frost builds up on the coil and fins thefrost tends to retard heat transfer between the coil and the air so thatas frost accumulates the cooling effect on air passing through the coilis reduced and the air being drawn into the scroll housing is at aprogressively higher temperature. This results in the density of the airmoved by the blower becoming progressively less which is also a factorin reducing the torque load. Therefore, in a sense the controlarrangement of this invention not only senses the reduction in air flowvolume due to frost accumulation but it is also, in a sense, temperaturesensitive thereby giving a further, or more accurate, indication of thefrost condition of the coil.

This invention is concerned with sensing the change in motor speed dueto the frost condition and initiating a defrost cycle at a control pointin the refrigeration cycle indicating that an excessive amount of frosthas accumulated on the evaporator coil. This is accomplished by couplinga speed responsive mechanism to the motor and, in the illustratedembodiment, energizing defrost heater 32 at the designated controlpoint. Although the speed responsive mechanism can take any of a numberof forms it is preferably a centrifugal switch assembly 34 which, in amanner to be described more completely hereinafter, is connected toshaft extension 36 of motor 28.

Heater 32 is positioned adjacent evaporator coil 16 and when energizedwill heat the coil to remove frost. The heater is connected to source 38through switch of the speed responsive assembly 34 and a temperaturesensitive switch 40. During the normal refrigeration cycle switches 35and 40 are in the positions illustrated, contact 41 on movable switchblade 42 of switch 35 in engagement with fixed contact 44 and movableswitch blade 46 of switch 40 engaging fixed cold contact 48. With thisarrangement the electrical circuit to heater 32 is open so that theheater is not energized during the refrigeration cycle. Tracing theelectrical circuit from L through junctions 50, 52, heater 32, contact48, switch blade 46 and lead 54 to fixed contact 56 of switch 35 it willbe seen that the circuit to L is open at switch 35 and therefore heater32 is de-energized. However, the appropriate circuits are completed forcarrying out the refrigeration cycle. More specifically, a circuit ismade from L through lead 56, terminal assembly 58 to movable switchblade 42 and from the movable switch blade through fixed contact 44,lead 60, thermostat 62, junction 64, start relay assembly 66 and lead 68to compressor 10 and L;,. A parallel circuit is also completed throughblower motor 28, namely from junction 64 through leads 70 and 72 to doorswitch 74 and through lead 76 to L When the door of the cooler isclosed, switch blade 78 engages contact 80 completing the just-describedparallel circuit. When the door is open, switch blade 78 is moved intoengagement with contact 82 thereby interrupting delivery of cold air bythe blower and automatically turning on lights 84 within the cooler.Accordingly, the necessary electrical circuits are made to the variousoperational elements to carry out the refrigeration cycle.

As the refrigeration cycle progresses frost accumulates on theevaporator coil. As a result of frost accumulation the amount of airwhich can be drawn through the evaporator coil is reduced therebyreducing the torque load on blower 20. As mentioned previously, thefrost accumulation also tends to insulate the air flow from the primarycooling members, namely the coil and fins, so that the temperature ofthe air entering scroll housing 22 is relatively higher as theaccumulation of frost progresses. The reduced volume of air and to alesser extent the increased temperature thereof, reduces the torque loadon blower 22 and produces a corresponding increase in the speed of motor28. At a preselected control point in the refrigeration cycle indicatingan excessive amount of frost accumulation on the evaporator coil,centrifugal switch assembly 34 is operated, in a manner to be describedmore completely hereinafter, to switch movable blade 42 and contact 41from contact 44 to contact 56. This switching action deenergizes theblower motor and compressor thereby terminating the refrigeration cycleand simultaneously energizes defrost heater 32 to initiate the defrostcycle. More specifically and with reference to the above describedcircuits, it will be seen that movement of switch blade 42 away fromcontact 44 opens the circuit through lead 60 to both motor 28 andcompressor 10 and, simultaneously, movement into engagement with contact56 completes the circuit through switch assembly 40 to defrost heater32.

The defrost cycle progresses with heater 32 melting the frostaccumulated on the evaporator coil. Any suitable, conventional liquidcollecting arrangement can be utilized with the evaporator coil tocollect and remove moisture resulting from the melting frost.

Any of a number of arrangements can be used to establish the terminationof the defrost cycle. As illustrated, temperature sensitive defrostswitch assembly 40 is used and is exposed to the temperature ambientevaporator coil 16 so as to monitor the defrost cycle. When apreselected control point in the defrost cycle is reached indicating thedesired amount of melting of frost from the evaporator coil, movableblade 46, which can be a bimetal member, of switch assembly 40 movesfrom cold contact 48 to hot contact 84, this switching action being inresponse to the temperature ambient the evaporator coil and interruptingthe circuit to heater 32 to initiate termination of the defrost cycleand set up a circuit for reestablishment of the refrigeration cycle. Ina more specific aspect of this invention it is proposed to provide atime delay between termination of the defrost cycle and reestablishmentof the refrigeration cycle. In the illustrated embodiment this timedelay is achieved utilizing a reset mechanism 86 associated withcentrifugal switch assembly 34. More specifically, reset mechanism 86includes bimetal member 88 connected to rod 90. Bimetal 88 is in heattransfer relation with reset heater 92, the reset heater being connectedin circuit with and controlled by defrost switch assembly 40. Withreference to switch assembly 40, it will be noted that when movableblade 46 is in engagement with cold contact 48, thereby establishing thedefrost heater circuit, the circuit to reset heater 92 is open. Whenmovable blade 46 is switched to hot contact 84 to initiate terminationof the defrost cycle, the circuit to reset heater 92 is completedthereby energizing heater 92 to straighten bimetal 98 and through rodswitch movable blade 42 from contact 56 back to contact 44 andreestablish the refrigeration cycle. This switching action toreestablish the refrigeration cycle is thus accomplished with asufiicient time delay to insure completion of the defrost cycle.

The structure of the illustrated centrifugal switch assembly will now beexplained. Movable blade 42 is part of an over-center or toggle switcharrangement which can be of any conventional construction. Asillustrated, movable blade 42 includes a pair of side rails 93 and 94,these side rails being connected to fixed posts 96 and 98 which providea pivot about which blade 42 is movable.

. The side rails are also connected to a rigid member 100 and throughthat member to a centrally located tongue 102. An over-center or togglespring 104 extends between tongue 102 and the forward or contactcarrying portion of blade 43. In a conventional manner, toggle spring104 holds the movable contact in engagement with either contacts 44 or56 as illustrated in FIGS. 1 and 2 and will cooperate in producingswitching action of the movable contact when the point of engagementbetween tongue 102 and spring 104 has been moved overcentcr with respectto rails 93 and 94. The centrifugal actuator for the just-describedtoggle switch includes a disk 106 which is connected to shaft 36 onblower motor 28. Disk 106 rotates with the shaft. A weight 108 isarranged at the periphery of wheel 106. Weight 108 includes a pair ofspaced arms 110 and 112 which straddle wheel 106 and are connected byweb 114. Each arm 110 and 112 is provided with an open slot 116 engagedon pin 118 fixed to wheel 106. A tension spring 120 has an end 122thereof engaged on web 114 and its opposite end 124 engaged on pin 126fixed to wheel 106. The tension spring biases weight 108 in acounterclockwise direction as viewed in the drawings. Engagement betweenweb 114 and the periphery of wheel 106 limits counterclockwise movementof the weight. Corner 128 of weight 8 projects radially outwardly fromWheel 106 and the projection is such that it clears turned end 130 ofrigid member 100 of the switch assembly as long as blower motor 28 isbelow a preselected speed. As the blower motor speed increases wheel 106will rotate correspondingly faster. Rotation of wheel 106 results in acentrifugal force acting on weight 108 tending to pivot the weightagainst the bias of spring 120 and in a clockwise direction, Spring 120overcomes this centrifugal force so long as the speed of blower motor 28is below a preselected value. At the control point in the refrigerationcycle indicating a defrost cycle is required, the speed of motor 28 willhave increased such that the centrifugal force exerted on weight 108overcomes tension spring 120 sufliciently to displace the weightangularly and position corner 128 thereof such that it will engageturned end 130 and move rigid member 100 upwardly as viewed in thedrawings. This movement of member 100 upwardly produces correspondingdownward movement of tab 102 moving spring 104 overcenter with respectto side rails 93 and 94 and movable blade 42 switches contact 41 fromcontact 44 to contact 56. As described above, this initiates the defrostcycle and also de-energizes the blower motor to thereby stop wheel 106and allow spring 120 to return weight 108 to the position illustrated inFIG. 1. Thus the switch assembly is reset as described above at the endof the defrost cycle and the centrifugal switch has been returned to itsrest position so that the refrigeration cycle will progress until thecontrol point is again reached indicating defrost is necessary.

With the just-described control arrangement a particularly effectivedefrost control is provided which responds to the increase or decreasein speed of the blower and its drive motor in accordance withaccumulation of frost on the evaporator coil. As stated above, inductionmotors, such as shaded pole and permanent split capacitor motors, offersatisfactory operating characteristics for use in this control but itshould be appreciated that any electric motor having the characteristicof changing speed with changing torque load can be used in the controlincorporating this invention. Assuming a substantially constant voltageinput to the motor, motor speed varies inversely with the torque load onthe motor; this can be shown by plotting motor speed against percentageof blockage in the coil, for a particular voltage motor speed increasesas blockage increases and, correspondingly, torque decreases. Theresponse to changes in air flow can be optimized by refinement inselection and design of the motor, blower and scroll housing. For thisreason the blower design mentioned above is preferred, i.e., withforward curved blades.

The control of this invention requires relatively simplified circuitryas compared to heretofore available control arrangements and does notrely for effective operation on the proper functioning of any mechanicalelements directly exposed to the environment of the evaporator coil.

Thermostat 62 is merely an auxiliary control thermostat which can beexposed to the temperature in the area to be chilled and relay 66 ismerely intended to insure starting of compressor 10.

Furthermore, defrost systems other than an electrical heater could beused without departing from the spirit or scope of this invention.

Although but one embodiment of the present invention has beenillustrated and described, it will be apparent to those skilled in theart that various changes and modifications may be made therein withoutdeparting from the spirit of the invention or from the scope of theappended claims.

I claim:

1. In a cooling system including a cooling coil and motor driven blowermeans for moving air through said cooling coil and wherein said coolingcoil is susceptible to accumulation of frost which obstructs air flowthrough said cooling coil, the combination of:

selectively energizable means for defrosting said cooling coil,

and control means connected to said motor driven b-lower means and saidmeans for defrosting said cooling coil, said control means beingdirectly responsive to the speed of said motor driven blower means and,in response to a preselected blower means speed corresponding to apreselected change in air flow through said cooling coil, operating saidmeans for defrosting said cooling coil to initiate a defrost cycle.

2. The combination of claim 1 wherein said control means include:

centrifugal switch means.

means connecting said centrifugal switch means to said blower means andtransmitting rotational movement of said blower means to saidcentrifugal switch means,

and wherein said centrifugal switch means responds to said rotationalblower means movement to initiate said defrost cycle at said preselectedblower means speed.

3. The combination of claim 1 wherein:

said motor driven blower means draws air through said cooling coil andincludes blower means and motor means driving said blower means,

the load on said blower means decreasing as frost accumulates on saidcooling coil and the speed of said motor means increases an amountcorresponding to the decrease in load on said blower means,

and said control means connected to and responsive to the speed of saidmotor means.

4. The combination of claim 1 wherein said control means is furtheroperative to continue said defrost cycle and at a preselected controlpoint in said defrost cycle being operative to initiate termination ofsaid defrost cycle and initiate reestablishment of a cooling cycle.

5. The combination of claim 1 wherein:

said means for defrosting said cooling coil comprises heating means,

and said control means initiates operation of said heating means to heatsaid cooling coil at a control point in the cooling cycle indicating achange in blower means speed corresponding to a preselected amount ofobstruction to air flow through said cooling coil due to frostaccumulation on said cooling coil.

6. The combination of claim 5 wherein:

said control means is operative at said control point in said coolingcycle to terminate said cooling cycle and initiate operation of saidheating means to initiate said defrost cycle,

and wherein said control means further includes means operable at acontrol point in said defrost cycle to initiate termination of saiddefrost cycle and initiate reestablishment of said cooling cycle, saidlast-mentioned means further operative to reestablish said cooling cyclewith a predetermined lapse of time after deenergizing said heatingmeans.

7. The combination of claim 1 wherein:

said means for defrosting said cooling coil comprises electrical heatingmeans,

and said control means includes speed responsive switch means having afirst position Opening an electrical circuit to said heating means and asecond position closing an electrical circuit to said heating means sothat said speed responsive switch means selectively controlsenergization and deenergization of said heating means, said speedresponsive switch means connected to said motor driven blower means and,at said preselected blower means speed, switching to said secondposition to energize said electrical heating means.

8. The combination of claim 7 including system means for activating saidcooling coil to produce a cooling cycle and wherein:

said speed responsive switch means also controls said system means andin said first position is operative to complete an energizing circuit tosaid system means and in said second position opens said energizingcircuit so that said cooling cycle and defrost cycle occuralternatively.

9. The combination of claim 8 including reset means operative at acontrol point in said defrost cycle to initiate deenergization of saidheating means, said reset means operatively associated with said speedresponsive switch means and operative to switch said speed responsiveswitch means to said first position and reestablish said cooling cyclewith a predetermined time delay after said control point is reached.

10. The combination of claim 9 including:

defrost termination and reset switch means responsive in circuit withand controlling said defrost heating means,

reset heating means in circuit with and controlled by said reset switchmeans,

said reset switch means, at said control point in said defrost cycle,opening the circuit to said defrost heating means and closing thecircuit to said reset heating means,

and thermoresponsive actuating means in heat transfer relationship withsaid reset heating means and engageable with said speed responsiveswitch means, said actuating means responding to said reset heatingmeans to switch said speed responsive switch means to said firstposition with said predetermined time delay. 11. The combination ofclaim 10 wherein: said motor driven motor means draws air through said 5cooling coil and includes blower means and motor means driving saidblower means, the load on said blower means decreases as frostaccumulates on said cooling coil and the speed of said motor meansincreases an amount corresponding to the decrease in load on said blowermeans, and said speed responsive switch means connected to andresponsive to the speed of said motor means. 12. The combination ofclaim 7 wherein said motor driven blower means includes blower means andmotor means connected to and driving said blower means,

said speed responsive switch means includes a movable contact carryingmember movable selectively to said first and second switch positions, arotatable member, and means connecting said rotatable member forrotation by said motor means so that the rotational speed of saidrotatable member corresponds to that of said motor means, said rotatablemember disposed adjacent said movable contact carrying member andincluding a weight portion and means supporting said weight portion onsaid rotatable disk and biasing said weight portion in a first positionwith respect to said rotatable member and supporting said weight portionfor radial outward movement against said bias and with respect to saidrotatable disk in response to the rotational speed of said rotatablemember, said movable contact carrying member in said first positionbeing disposed out of the path of rotation of said weight portion whensaid weight portion is in its first position and said weight portionmoving radially for engagement with said movable contact carrying memberas the rotational speed of said rotatable member increases.

MEYER PERLIN, Primary Examiner.

